1. Field of the Invention
This invention relates to a jet pump that can be installed in an automobile fuel tank where it utilizes the flow of return fuel from the engine as operating fluid for moving fuel from a sub fuel tank to the main fuel tank.
2. Description of the Prior Art
Automobile fuel tanks are usually located in the rear part of the car. Because of the existence of the drive shaft, differential gear and so forth, as shown in FIG. 7, in some cases the center part of the fuel tank t has to be curved inward, giving the fuel tank t a saddle shape. In this case, the tank t is divided into a main tank m in which the fuel pump p is located, and a sub-tank s, so that when there is not much fuel left, fuel in the sub-tank s cannot be fed to the engine.
One arrangement to remedy this comprises installing a jet pump j in the tank t that uses return fuel from the engine as its operating fluid, with the jet pump j being used to move fuel from the sub-tank s to the main tank m. FIG. 6 shows an arrangement of a prior art jet pump j, as disclosed by Japanese Patent No. 2598091. With reference to FIG. 6, the jet pump includes a jet nozzle a that is used to jet return fuel from the engine, a chamber b that encloses the jet nozzle a, an intake pipe e for drawing fuel from the sub-tank s into the chamber b, and a throat pipe d through which, via a constriction portion c at the tip of the jet nozzle a, the return fuel and the transfer fuel is fed to the main tank m.
Return fuel is the excess fuel that has been delivered to the engine by the fuel pump p(see FIG. 7) and is returned to the fuel tank t. In the jet pump j, this return fuel is jetted from the jet nozzle a toward the throat pipe d in the chamber b. The negative pressure, or entrainment pressure, thus generated causes fuel in the sub-tank s to be sucked into the chamber b via the intake pipe e. The transfer fuel from the sub-tank s, together with the return fuel, is discharged into the main tank m from the throat pipe d, thereby transferring fuel from the sub-tank s to the main tank m.
In this case, in order to develop a sufficient negative pressure in the chamber b, it is necessary to effect a liquid seal of the liquid current (fuel flow) in the constriction c, particularly during starting for developing a negative pressure. In the prior art jet pump j, as shown in FIG. 6, this is handled by providing a swing plate f in the jet nozzle a. The swing plate f forms the liquid seal by spreading out the flow of return fuel streaming from the jet nozzle a (as indicated in FIG. 6 by the dot-dash line g).
However, in this prior art jet pump j, the swing plate f used for spreading the jetted stream g of return fuel causes a pressure loss. Because the angle of divergence of the jetted stream g cannot be increased by increasing the angle of the swing plate f, there is a limit to how much the jetted stream g can diverge, so the constriction c and throat pipe d have to have relatively small inside diameters. Thus, the flow of transfer fuel obtained with the jet pump j is not necessarily enough, so there has been a need to improve the transfer flow.
Moreover, factors such as temperature elevation within the chamber b causes cavitation, the forming of bubbles that impede the fuel flow in the vicinity of the inside wall of the throat pipe d. During cavitation, the actual flow path is confined to the center of the throat pipe d, and this, added to the fact that the throat pipe d has to be given a small inside diameter, makes it difficult to secure an adequate flow path during cavitation.
In view of the above drawbacks of the prior art, an object of the present invention is to provide a jet pump that can provide a good liquid seal during starting to thereby generate a sufficient negative pressure, in which the chamber constriction portion and throat pipe do not have to be given particularly small diameters but can be given optimum diameters, making it possible to ensure a sufficient transfer flow, and which enables reduction of the transfer flow during cavitation to be minimized.
To attain the above object, the present invention provides a jet pump, comprising a jet nozzle that discharges an operating fluid, a chamber that encloses a tip of the jet nozzle and has an internal space into which a transfer fluid flows, a throat pipe having a constriction portion via which the chamber is narrowed from a vicinity of the jet nozzle tip that discharges the transfer fluid that flows into the chamber, the throat pipe formed to have a discharge end with a smaller diameter than that of a base end, with a ratio between base-end inside diameter D1 and discharge-end inside diameter D2 of the throat pipe being set at D1/D2=1.01-2 and the smaller-diameter portion being bent at an angle of approximately 90 degrees from the base end.
Thus, in the jet pump of this invention, by forming the throat pipe so that the tip has a smaller diameter than the base end and with the smaller-diameter portion having a bend of around 90 degrees from the base end. This ensures that a liquid seal is secured, particularly during starting, generating sufficient negative pressure to ensure a good flow rate of the transfer fluid, and does not involve restrictions such as having to decrease the inside diameter of the chamber throat constriction portion and throat pipe.
Because in the jet pump of this invention the tip of the throat pipe has a smaller diameter than the base end and the smaller-diameter portion is bent at 90 degrees to the base end, when an operating fluid such as return fuel is jetted at the throat pipe from the jet nozzle during starting, the operating fluid dwells momentarily inside the throat pipe, submerging the tip of the jet nozzle to thereby form a liquid seal between the jet nozzle and the chamber constriction and throat pipe base-end portion. The good negative pressure generated by means of this liquid seal enables a good flow of transfer fuel or the like to be achieved. The ratio between the base-end inside diameter D1 and discharge-end inside diameter D2 of the throat pipe is set at D1/D2=1.01-2, and more preferably is set at D1/D2=1.05-1.2. By effecting dwelling of the operating fluid during starting, this ensures a good liquid seal and, during the transfer phase following the generation of the negative pressure, enables a good fluid flow rate to be achieved without any deterioration of fluid flow properties in the throat pipe.
The inside diameters of the constriction portion and throat pipe can be optimized for the diameter of the jet nozzle used, to minimize pressure loss over the range in which a good liquid seal can be achieved, maximizing the transfer flow rate that can be achieved under the conditions prescribed. Also, since the constriction and throat pipe can be given sufficiently large diameters to ensure an adequate flow path even during cavitation, it is possible to maintain a good transfer flow rate.
While not limitative, it is desirable for the jet nozzle to be a straight nozzle of a prescribed length and hole diameter that will enable the fluid to be discharged in a straight jet without divergence, since during cavitation this will enable the fluid to flow along the center of the throat pipe without being affected by the cavitation, thereby making it possible to maintain a sufficient transfer fluid flow even when there is cavitation.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.